A liquid flowing through a capillary jet or orifice may be converted into a spray of small charged droplets (on the order of I micrometer in diameter) by applying an electric field to the liquid as it emerges from the tip of the capillary. For a sufficiently high applied electric field, the electrostatic stress imposed by the field and the surface-induced electric charge is sufficient to overcome the surface tension forces on the liquid. The liquid breaks apart into small charged droplets. This process of forming a spray is known as electrospray.
Electrospray is widely used for analysis of sample solutions. For example a sample solution such as a liquid stream effluent from a liquid chromatography (LC) separation step is atomized by an electrospray device and analyzed with a mass analyzers such as a quadrupole mass spectrometer, an ion trap mass spectrometer, a time-of-flight mass spectrometer, or a magnetic sector mass spectrometer. Electrospray ionization mass spectrometry is also widely used for the analysis of biological molecules, including peptides and proteins.
An example of a prior art electrospray apparatus is described in U.S. Pat. No. 5,572,023 issued to Caprioli. Caprioli describes an electrospray apparatus and method including an electrically charged capillary spray needle which may be filled with packing material forming a column bed. The packing material differentially adsorbs selected chemicals in the sample solution before it is discharged from the spray needle into the vaporizing and analysis chamber. Caprioli discloses charging the sample solution at an upstream location by passing it through a steel "zero dead-volume" fitting. The steel fitting is connected to a high voltage source, thereby imparting a charge to the sample solution. The charged solution then continues through tubing to the non-conductive spray needle and is discharged. This conductive fitting is located substantially upstream from the discharge end of the spray needle. As reduced to practice, the voltage source must always be placed upstream of the column bed.
A number of problems are caused by this setup. First is a requirement for excessive dead volume. "Dead volume," as used in Caprioli, is that volume outside the column bed through or into which the solution sample must flow or diffuse. Longer flow paths cause excess dead volume, thereby requiring more sample solution to fill the dead volume, and also results in bandspreading in a chromatographic analysis.
Caprioli addresses the issue of postcolumn dead volume, which leads to bandspreading, but ignores that of precolumn dead volume and holdup volume. Precolumn dead volume is the volume before the column bed, and holdup volume is the system volume between the point of gradient generation and the front of the column bed. Precolumn dead volume results in bandspreading, specifically when present in isocratic HPLC (High Performance Liquid Chromatography) methods. Excessive holdup volume, together with excessive precolumn dead volume, results in a longer run to run turnaround time, especially (but not exclusively) with gradient HPLC methods.
The electrical contact in Caprioli is upstream of the column bed. The transport tubing to the column is noncontinuous (severed) in order to provide electrical contact with the sample solution. This in turn necessitates the use of a leakproof joint capable of withstanding the high fluid pressure generated by the column bed. Such joints are troublesome, as shown in the embodiment. While Caprioli employs a conventional "zero dead volume" fitting, this term is unclear because the fitting clearly introduces dead volume. The means by which the two 50 micron ID (inside diameter) tube orifices are mated are not described specifically, but it is safe to assume that it was done in a conventional manner, using a PEEK sleeve, similar to the needle support. The OD (outside diameter) of the tubing used varies from 140 microns to 350 microns. This is well below the through hole of the fitting, specified at 0.5 mm (500 microns). In any scenario, it is extremely difficult, if not impossible, to make a truly "zero" dead volume connection. The result is an unpredictable contribution to precolumn bandspreading.
Further, as disclosed in Caprioli, when the electrospray electrode is located significantly upstream of the needle tip and column bed, the electrical resistance between the electrode and the needle tip becomes significant, especially with smaller capillary inner diameters. This means that an excess potential must be maintained on the electrode relative to the resulting potential at the needle tip. Undesirable electrical arcing and corona discharge in the electrode region can occur.
Still further, in a given LC/ESI/MS system, if the electrode is moved further from the needle tip and upstream of the column, it is necessarily placed closer to the injector and pump. This in turn decreases the electrical resistance between the electrode and these system components, causing more electric current to flow to them. This presents one of two problems. If the component is not grounded it, like the electrospray tip, will float at some voltage less than that of the electrode, creating the operational and safety problems associated with the abrupt discharge of high voltage (arcing). If the component is grounded, a substantial current will flow through the component which may exceed the current limits of the power supply. The solution to this problem, as disclosed by Caprioli, is to increase the resistance between these components and the electrode by using longer lengths of tubing between the pump and injector, and/or between the injector and electrode. This extra tubing results in a cumulative increase in holdup volume and/or precolumn dead volume, as previously discussed. Again, this implies more bandspreading in the case of isocratic operation, and longer turnaround times in the case of gradient operation.
Still another problem known in the art is presented by the metallic electrodes commonly employed internal to electrospray sources. It has been observed that electrochemically active compounds may react at the surface of some metallic electrodes. In the case of electrospray mass spectrometry, this results in a decrease in ion intensity for the target compound and/or the appearance of ions produced from the products of the oxidized or reduced target compound. Additionally, components of the mobile phase may form ionic complexes with metallic components of the electrode. Such organometallic complexes then interfere with mass spectral measurements. If the electrode is placed between the injector and the column, for example by the use of a metallic fitting, compounds swept across the surface of the electrode are subject to such interactions.